He made the announcement after Philip Hammond said that it appeared that the Islamic State extremist who beheaded Mr Foley is British.

The Foreign Secretary said the Government was "absolutely aware" that significant numbers of British nationals have been involved in "terrible crimes, probably in the commission of atrocities" in the conflict.

Mr Hammond said that Britain and US intelligence officials will be analysing the video to try to identify the jihadist.

He warned that Britons could return and use the "skills and tradecraft" learned from the terrorists to pose a direct threat to "our domestic security here in the UK."

Islamic State jihadists have released a video in which a militant speaking English with a London accent beheads a man they claim is American journalist James Foley who was kidnapped in Syria two years ago.

Mr Hammond confirmed that British troops could now be sent into Baghdad to train Iraqi forces but insisted there was still no need to recall parliament to allow MPs to debate Britain's involvement in the conflict.

'Horrifying'

Britain would continue to oppose the "horrifying and shocking" force of Isis "with every breath in our body”, he added.

He told the BBC: “Some more analysis to make quite certain that is the case. But we have been saying for a very long time there are significant numbers of British Nationals in Syria, increasingly in Iraq.

“And one of the reasons why what is going on in Syria and Iraq is a direct threat to our own national security is the presence of significant numbers of our nations who may at some stage seek to come back to the UK with the skills, the tradecraft that they have learned working with these terrorist organising are potentially posing a threat to our domestic security here in the UK.”

Asked if Britain was “prepared to send soldiers” to help train Iraqi forces fighting Isis said it was “certainly” under consideration.

“The training doesn’t necessarily have to be done in country … but equally we could decide to put limited numbers of trainers into Baghdad for examples, we could train Iraqi trainers to go out and train Iraqi security forces, we absolutely are prepared to consider requests for technical assistance, training support, advice and so on.

He added that that surveillance flights being carried out for humanitarian reasons could “in principal” be extended in support of Iraqi forces efforts to push back Isis fighters.

Mr Hammond said “many” British nationals join Isil by travelling through third countries like Turkey “which is right next door to Syria” under the guise of going on holiday.

He added: “I’m not able to give you figures for how many people we’ve stopped, how many people we’ve dissuaded, how many people we’re currently monitoring for obvious reasons.

"But there is a major operation under way to ensure that we do everything we can to reduce the numbers going, to monitor those who do go and to intercept as many as possible of them if and when they seek to come back.”

Propaganda

Experts have warned that images from propaganda videos, such as the beheading, and direct contact with young British people who have travelled to Iraq “humanises” the fight for those who are already attracted to the conflict.

Shiraz Maher, senior research centre for radicalisation studies at King’s College London said that British involvement in both Syria and Iraq has been “one of full participation” with Britons taking part in the conflict in “every way”.

“We have seen British fighters out there are acting as suicide bombers, operating as executioners,” he said.

He added: “We are also seeing on social media the ability of people who are out there to communicate without others back in the west to say I am just like you. I was 18 or 20, I have just done my A-levels, if I can make it here you can make it here and that element of direct interaction with fighters ... they have been humanised with their experiences out there with the people who look up to them.

He said while films such as the beheading “dehumanised” Isis to the rest of us, "to those who are attracted to this idea it makes it makes it that much more real and feasible.”